(Reuters) - Southwest Airlines Co said it expects an important revenue measure to decline in the second quarter, saying it is having to work harder to stimulate traffic.

Chief Financial Officer Tammy Romo told a Deutsche Bank investor conference on Thursday that passenger revenue per available seat mile, which is also known as unit revenue, would fall in the low-single-digit percentage range in the current quarter compared with a year ago.

"Revenue trends since March have been softer than we anticipated," Romo said during the conference, which was broadcast over the Internet.

Unit revenue at Southwest fell 4 percent to 5 percent in April and was down about 2 percent in May.

Romo added that while bookings for June were good, the company was cautious about summer demand and potential effects from automatic U.S. budget cuts and higher taxes.

Shares of Southwest were down 0.4 percent to $13.82 in morning trading.